Metal halide high intensity discharge (HID) lamps are desired to run at high wall temperatures in order to improve the efficacy, alter the color temperature, and/or raise the color rendering index of the light source. Typically, the metal halide lamps include fills comprising halides (especially iodides and bromides) of one or more metals, such as Na. Often Na is used in combination with Sc or Sn. Further additions are Th, Tl, In and Li. Other types of filling include rare earth metals such as Tm, Ho and Dy. Lamps which contain such fills have very desirable spectral properties: efficacies above 100 lm/W, color temperatures of about 3700 K, and color rendering indices (CRI) around 85. Because of the low vapor pressure of some of the metal halide additives, the fused quartz lamp envelope must be operated at higher than normal temperatures. At wall temperatures exceeding 900.degree.-1000.degree. C., the lifetime of the lamps is limited by the interaction between the metal halides and the wall made from quartz glass. The use of arc tube materials which can be operated at higher temperatures than quartz glass and which are chemically more resistant than quartz glass provides an effective way to increase the lifetime of lamps containing these metal halides.
Polycrystalline alumina (PCA) is a sodium resistant envelope for high pressure sodium lamps. PCA can operate at higher temperatures than quartz glass and it is expected to be chemically more resistant than quartz glass. The PCA vessel is closed at its ends by means of alumina plugs. Gastight sealing is achieved by sealing glass, often referred to as fusible ceramic or frit. However, investigations of metal halide chemistries in PCA envelopes have shown that reactions between the metal halides and conventional frits or even allegedly "halide-resistant" frits severely limit lifetime. An example of such a frit is based on the components CaO, Al.sub.2 O.sub.3, BaO, MgO and B.sub.2 O.sub.3. Consequently, it is highly desirable to find a fritless seal method.
Normally, PCA lamps use feedthroughs made from niobium because their coefficients of thermal expansion are similar. Especially when the fill contains rare earth halides, one problem is involved by the reactions between the Nb feedthroughs and the fill. This problem was alleviated somewhat by using special arrangements wherein the plug and the feedthrough is simultaneously replaced by a plug made from electrically conductive cermets. These cermets are composite sintered bodies usually comprising alumina (the arc tube material) and molybdenum (Mo) or tungsten (W), both metals being halide resistant materials.
U.S. Pat. No. 4,354,964, Hing et al., discloses an electrically-conducting alumina-metal (e.g. tungsten or molybdenum) cermet containing 4 to 20 vol. % metal for use as plug members or feedthroughs in PCA (polycrystalline alumina) envelopes of metal halide HID (high-intensity discharge) lamps. The cermet has refractory metal rods (as electrodes or current leads). They are embedded in the cermet body in the green or prefired state and then co-fired during final sintering of the cermet to high density. The method of joining such cermets with PCA tubes is not described. Thermal expansion mismatch between the cermet and PCA, or between the cermet and tungsten or molybdenum electrode can not be eliminated simultaneously. Such differential thermal expansion can result in cracking and leaks in either PCA tubes or cermet, or in both, during lamp on-and-off operation.
U.S. Pat. No. 4,731,561, Izumiya et al., shows one end of the PCA tube that is closed with a co-sintered electrically-conductive alumina-Mo or W cermet. The other end of the PCA tube is enclosed with a frit-sealed cermet. The cermets are all coated with an insulating layer so as to prevent back-arcing.
U.S. Pat. No. 4,687,969, Kajihara et al, describes besides conducting cermet plugs also non-conducting cermets with feedthroughs passing through and projecting in- and outwardly. One end of the PCA tube has a co-sintered cermet, while the other end has a frit-sealed cermet. However, cracking in the cermet can not be prevented, since the composition of the plug is fixed and is not direction dependent.
All these one-part plugs have the disadvantage that their coefficient of thermal expansion does not really fit the surrounding part (e.g. the vessel). A solution is suggested for example in U.S. Pat. No. 4,602,956, Partlow et al. It discloses a cermet plug that comprises a core, consisting essentially of 10 to 30 volume percent W or Mo, remainder alumina, and one or more layers of other cermet compositions surrounding the core and being substantially coaxially therewith. The layers consist essentially of from about 5 to 10 volume percent W or Mo, the remainder alumina. Such a cermet plug is hermetically sealed to the end wall of the arc tube by means of "halide-resistant" frits.
However, electrically conductive cermet plugs are not sufficient gastight over a long time due to their fine structure.
Another solution is a non-conductive cermet plug having a more dense structure. Consequently, a separate metal feedthrough is needed. U.S. Pat. No. 5,404,078, Bunk et al., discloses a high pressure discharge lamp with a ceramic vessel whose ends are closed by non-conductive cermet plugs consisting for example of alumina and tungsten or molybdenum. In a specific embodiment (FIG. 9) the cermet plug consists of concentric parts with different proportions of tungsten. These parts provide gradually changing coefficients of thermal expansion.
European Patent Application No. 650 184, Nagayama, discusses an arc tube with end plugs consisting of a non-conducting cermet. The cermet plug is made from axially aligned layers of different composition (axially graded seal, see FIG. 16 et seq.). The first layer of the plug is integrally attached to the open end of the vessel. The metal feedthrough is a tungsten-based rod. The sealing between the feedthrough and the last axially aligned layer of the plug is performed by a rather complicated technique. It uses
a threaded portion of the feedthrough being in direct contact with the last layer of the plug, PA1 an outer metal disc ("flange") in contact with the outer surface of the last layer PA1 and a sealant such as platinum or glass solder covering the flange and the outer surface of the last layer. PA1 a translucent ceramic tube having a first end and a second end, said tube defining a longitudinal axis, and said tube confining a discharge volume PA1 a first at least essentially electrically non-conducting cermet end plug, said first plug closing said first end of the ceramic tube PA1 a second at least essentially electrically non-conducting cermet end plug, said second plug closing said second end of the ceramic tube PA1 at least said second plug having a multipart structure with at least three parts PA1 a first and second metal feedthrough passing through the first and second plug respectively, each feedthrough having an inner end and an outer end, respectively, said feedthroughs being made from one of the group of the metals tungsten, molybdenum and rhenium and alloys from at least two of these metals PA1 two electrodes located at the inner end of the first and second feedthrough respectively PA1 the coefficient of thermal expansion of at least one part of the multipart plug being between those of the arc tube and the feedthrough PA1 wherein said multipart plug is a structure that comprises at least four axially aligned parts with different coefficients of thermal expansion, including a first and a last part, the first part being innermost with respect to the discharge volume and the last part being outermost with respect to the discharge volume PA1 the multipart plug is directly sintered both to the arc tube and the feedthrough in that manner that the innermost part of the plug is directly sintered to the arc tube and the outermost part of the plug is directly sintered to the feedthrough. PA1 (1) a uniform distribution of the dispersed phase, PA1 (2) a fine particle size of the dispersed phase, PA1 (3) a green density and firing shrinkage compatible with the neighboring layers, in order to produce graded cermets free of cracks or distortion, PA1 (4) a green density and firing shrinkage behavior so as to form a direct bond between metal feedthrough and cermet plug, and between cermet plug and PCA arc tube, respectively. PA1 outside diameter: 3.0 to 4.0 mm (with the proviso that possibly the first part has a greater diameter), PA1 length over all of the axially graded plug: up to 10 mm, preferably below 5 mm, PA1 outer diameter between 0.9 and 1.6 mm PA1 inner diameter between 0.6 and 1.2 mm PA1 over all length between 10 and 15 mm PA1 wall thickness is at most 0.25, preferably around 0.1 mm. PA1 There is absolutely no frit in the seal, but nevertheless a well established and very reliable sealing technique, direct sintering, can be used. PA1 The fill hole can be large enough to permit easy pumping and filling. PA1 This type of sealing works for any wattage of the lamp and for any size of the discharge vessel.
One of the rods acting as a feedthrough has an axial hole therein for inserting the fill into the discharge vessel.
U.S. Pat. No. 4,155,758, Evans et al., discloses in FIG. 14 an axially graded plug, too. However, it is made from three layers of electrically conducting cermet.